1. Field of the Invention
The present invention generally relates to a tempered glass and a method and an apparatus for quenching a glass sheet to obtain the tempered glass and more particular, the present invention relates to a thin tempered glass having a large surface area and a complicated curved surface i.e., such as a back light glass for automobiles, and a quenching method and a quenching apparatus for tempering the glass sheet of reduced thickness.
2. Discussion of the Background
A tempered glass is often used for a window glass for the side and rear windows of automobiles, but not for a front windshield glass. Tempered glass for automobile windows must meet official safety regulations with respect to fragmentation to prevent injury to a driver or a passenger of the automobile and to be permissible for use in the automobile.
For example, in one of the regulations on the tempered glass for automobile windows, there is a regulation concerning a state of fragmentation of glass produced when a localized impact is given to the tempered glass. Specifically, an area in which a number of the fragments of a glass sheet broken by an impact is minimum and an area in which a number of the fragments is maximum are selected, and the minimum and maximum numbers of the fragments in these areas have to fall in permissible ranges. The maximum size of glass particles produced from a fractured glass sheet is determined from a minimum permissible number of the fragments. When the maximum size is small, a danger of suffering injury from larger fragments is reduced. Further, the minimum size of the fragments produced by the fracture of the glass sheet is determined by a maximum permissible number of the fragments. When the minimum size is large, a danger of entering of glass particles into a human body is reduced. ECE standards or JIS standards rule the magnitude and so on of fragmentation of glass sheet when fractured. In ECE standards (E6), for example, it is required that a number of fragments in any 5 cm.times.5 cm square should be 40 at the minimum and 400 at the maximum (except for a belt-like region of 20 mm from the edge of the glass sheet and a circular region of 75 mm radius having the center which is the point of initiating breakage). In the following description, the maximum value of a number of fragments of glass is referred to as the maximum number and the minimum value is referred to as the minimum number. Further, there are requirements that when a glass sheet is broken, edges of fragments should not be sharp and elongated fragments having a length of 75 mm or more should not be produced. Further, there is a requirement that the surface area of a fragment should not exceed 3 cm.sup.2.
A tempered glass can be formed by heating a glass sheet to a temperature near the softening point of the glass (usually about 600-700.degree. C.) and quenching it by supplying cooling air. The cooling air is blown to the glass sheet through a plurality of cooling nozzles disposed near both surfaces of the glass sheet. Thus, a temperature difference is given to the glass sheet between a surface portion and the inner portion of the glass sheet at the time of quenching so as to from a compressive stress layer in the glass surface finally solidified, whereby the glass sheet is tempered.
Recently, weight reduction is required for automobiles to reduce fuel cost and so on. With this, there is an increased demand of reducing the weight by reducing the thickness of glass sheets. Using a glass sheet of about 4-6 mm thick, a tempered glass satisfying the above-mentioned requirements can easily be obtained by the above-mentioned glass tempering method (quenching method). However, when a thin glass sheet is to be formed to meet the requirement of weight reduction, it was difficult to obtain a tempered glass satisfying the regulations by the above-mentioned tempering method because a sufficient temperature difference could not be formed between the surface and the inner portion of the glass sheet due to the glass sheet being thin.
In concepts, there are considered various measures to increase a pressure of cooling air; to bring the nozzles closer to the glass sheet; to reduce the distance (pitch) between nozzles and so on in order to provide a sufficient temperature difference between the surface and the inner portion of the glass sheet. An attempt of increasing a pressure of cooling air is not realistic because there is a limit in terms of mechanism in a blowing device or a compressor.
It is necessary that the cooling air is supplied to the glass sheet to assure a way of escape of the cooling air after it impinges on the glass sheet. If the cooling air after impingement stay there, the cooling air prohibits successively supplied cooling air from impinging on the glass sheet whereby it is difficult to obtain uniform blowing of cooling air to the glass sheet. When the nozzle pitch is reduced or the nozzles are brought closer to the glass sheet, the way of escape of cooling air after the impingement on the glass sheet can not be assured.
Further, there has been proposed a method of oscillating the glass sheet at the time of blowing cooling air for tempering the glass sheet, whereby the glass surface is uniformly quenched. In this method, when the nozzles are brought closer to the glass sheet, the oscillated glass sheet may interfere with the nozzles. In particular, when the glass sheet is shaped to have a complicated curved surface, there is a large possibility of interfering of the nozzles with the glass sheet.
There has been proposed to conduct a tempering treatment with a special arrangement of nozzles so that a tempered glass of thin thickness can be obtained. The proposal is to control the propagation of fracture of glass by forming areas of different principal stress in the glass sheet.
Here, description is made as to a direction of the principal stress and a principal stress difference in a glass sheet. First, a plane which is in perpendicular to the glass sheet surface (a cross-sectional plane of the glass plate) is selected from the glass sheet, and then, a point is selected from the selected plane. The selected plane is selectable various angles with a line in parallel to the glass surface. Stresses of perpendicular direction of the selected plane acting on this point are unequal depending on angles of the selected plane. So, there is one selected plane which has the largest stress and the smallest stress, which are perpendicular each other, when a certain angle is selected among the various angles. The principal stress direction is defined the direction of the largest stress and the smallest stress. Hereafter, the direction of the largest stress is referred to as the principal stress direction, as representative. Further, the largest stress and the smallest stress (i.e., the stress in the direction perpendicular to the direction which indicates the largest stress) is a principal stress difference. In a tempered glass, the principal stress is estimated the principal stress difference which is obtained with photoelasticity method. The principal stress difference of the tempered glass is corresponds to a value obtained by dividing the sum of values of the difference between the largest stress value and the smallest stress value at points aligned in the glass sheet thickness direction by the thickness of the glass sheet (an average value obtained by dividing an integrated value of difference between the largest stress value and the smallest stress value by the thickness). Namely, when a certain point is selected in a surface of the glass sheet, an averaged integrated value of difference between the largest stress value and the smallest stress value at points aligned in the direction of the thickness from the selected point, is referred to as the principal stress difference at the selected point (the principal stress direction in this case is referred to as the principal stress direction at this point).
For the tempered glass in which there are areas having different principal stress direction, the following proposal is made. U.S. Pat. No. 4,128,690 describes a tempered glass having a thickness of 2.5-3.5 mm. The tempered glass has a central tensile stress of 62 MN/m.sup.2 at the maximum (a surface compressive stress of 124 MN/m.sup.2 at the maximum). The tempered glass has a distribution of areas in which the principal stresses acting in the plane of the glass sheet are unequal. Further, the US patent describes that in the areas having different principal stresses in the tempered glass, the maximum value of principal stress difference is in a range of 8-25 MN/m.sup.2 and the distance between the adjacent areas indicating the maximum value is in a range of 15-30 mm.
However, when the glass sheet having a thickness of 3.0 mm or less is actually produced as a tempered glass having the above-mentioned distribution, the following disadvantage is found. In the fragmentation test according to E6, the difference between a maximum number and a minimum number becomes large. This shows that either an upper limit of the maximum number or a lower limit of the minimum number ruled in E6 is apt to be outside even by a slight change of conditions for forming the tempered glass (e.g., an outside air temperature and so on). The tempered glass having such distribution tends to produce elongated fragments of glass. Further, the maximum surface area of the fragments is generally apt to exceed 3 cm.sup.2. It is supposed that such tendencies are derived from a coarse distribution of the areas.
Japanese Unexamined Patent Publication JP-A-55-104935 describes a tempered glass of 2.5-3.5 mm thick. The tempered glass has an average surface compressive stress of 850-1350 kg/cm.sup.2 and areas in which the principal stresses acting in the plane of the glass sheet are unequal are formed in a scattered state. In such areas, the maximum value of the principal stress difference is in a range of 50-300 kg/cm.sup.2, and the distance between adjacent areas indicating the maximum value is in a range of from 5 mm or more to less than 15 mm.
In the above-mentioned publication, there is a statement concerning a tempered glass of 2.5 mm thick in Example 5. The distance between adjacent areas indicating the maximum value of principal stress difference is 7.1-9.0 mm. Namely, it is understood that use of a thin glass sheet can obtain a tempered glass capable of meeting the official requirements if the above-mentioned distance is reduced. The reduced distance causes an irregular pattern of cracks in the tempered glass when the fragmentation test is carried out. The irregular pattern of cracks is advantageous in obtaining smaller fragments of glass.
However, in forming the irregular pattern of cracks, the development of cracks depends on nothing, namely, it is difficult to artificially control the production of cracks. On the other hand, a slight change in the conditions for forming the tempered glass will result in a delicate change of a magnitude of the stresses or a distribution of the stresses in the tempered glass to be produced. In particular, a thin glass sheet is easily influenced by a slight change of the conditions. Accordingly, if the development of cracks can not be well controlled, it is difficult to estimate a magnitude of the stresses or a distribution of the stresses whereby determination of the forming conditions is difficult.
Japanese Unexamined Patent Publication JP-A-58-91042 describes a tempered glass having a thickness of 2.4-3.5 mm in which belt-like regions having a higher surface compressive stress of 1300 kg/cm.sup.2 or less at the maximum and belt-like regions having a lower surface compressive stress of 1020 kg/cm.sup.2 or more at the minimum value are alternately formed in its surface. The difference between the maximum value and the minimum value of the surface compressive stress is 80-220 kg/cm.sup.2. In the belt-like regions having a lower surface compressive stress, the maximum value of the principal stress difference is 80 kg/cm.sup.2 or more.
Of several kinds of tempered glass described in the publication, attention is paid to a glass sheet of 2.4 mm thick. For such glass sheet, it is required either to reduce the width of the belt-like regions having a lower surface compressive stress or to increase the principal stress value of the belt-like regions having a lower surface compressive stress. When the width of the belt-like regions having a lower surface compressive stress is reduced, the irregular pattern of cracks can easily be produced. When the principal stress value of the belt-like regions having a lower compressive stress is increased, a practically required strength of the tempered glass may not be obtained because the principal stress difference is originally a tensile stress.
With respect to the practically required strength, the following disadvantage is, in particular, thought. Namely, the belt-like regions having a lower surface compressive stress correspond to portions to which cooling air streams are not applied during the tempered treatment. Accordingly, for the glass sheet having a thickness of 2.4 mm, it is practically difficult to render the surface compressive stress of glass sheet portions to which cooling air streams is not applied, to be 1020 kg/cm.sup.2 or more. Accordingly, it is estimated that the surface compressive stress in these portions is actually about 900 kg/cm.sup.2 at the most. The obtained value is insufficient in terms of a practically required strength in the tempered glass to be produced.
In an example of the above-mentioned publication, there is described an average surface compressive stress concerning a glass sheet having a thickness of 2.4 mm, i.e., there is described an example of a tempered glass having an average surface compressive stress of 1100 kg/cm.sup.2 and a difference 190 kg/cm.sup.2 between the higher and lower surface compressive stresses. The values disclosed therein is an average value between a higher compressive stress and a lower compressive stress. Accordingly, the publication does not describe a tempered glass in which the surface compressive stress in the belt-like regions is, in fact, 1020 kg/cm.sup.2.
As described above, the tempered glass having a thickness of about 2.5 mm in which a distribution of principal stress is formed in the glass sheet is known. However, it was in fact difficult for the tempered glass to satisfy the official requirements when a glass sheet having a thickness of about 2.5 mm was used. Further, there were many problems for equipment in order to obtain the thin tempered glass meeting the official requirements. In particular, there were problems of equipment for a glass sheet having a complicated curved shape or a glass sheet having a large surface area. In concept, the official requirements will be satisfied by making a distribution of areas in which the principal stresses are different to be dense. For the satisfaction, it is necessary to use measurements such as bringing the nozzles for supplying cooling air closer to the glass sheet, reducing the distance (pitch) between adjacent nozzles and so on. Such measures result the before-mentioned disadvantage that the way of escape of cooling air after impingement with the glass sheet can not be assured.